Polyester base film for magnetic recording media

Abstract

A biaxially stretched polyester base film having many fine protrusions on its surface, characterized by having the following properties: (a) in a distribution curve representing the relation between the number of protrusions (y/mm 2 ) and the height of protrusions (x in μm) on the surface of said film which are measured in a region having at least 20 protrusions/mm 2 , that part of the distribution curve at which the protrusion height (x) is larger than its peak value crosses the line of log 10 y=-18x+3.7, (b) when the number of protrusions (y) on the film surface is at least 20/mm 2 , no protrusion exists which has a height (x) of at least 0.13 μm, (c) said film has a surface roughness (Ra) of not more than 0.012 μm, (d) said film has a Young's modulus in the longitudinal direction of at least 650 kg/mm 2 , and (e) when heat-treated at 70° C. for 1 hour under no load, said film has a heat shrinkage in the longitudinal direction of not more than 0.1%.

What is claimed is: 
     
       1. A biaxially stretched, single-layer polyester base film containing inert solid fine particles dispersed therein and having may fine protrusions on its surface, said film having the following properties: (a) in a distribution curve representing the relation between the number of protrusions (y/mm 2 ) and the height of protrusions (x in μm) on the surface of said film which are measured in a region having at least 20 protrusions/mm 2 , that part of the distribution curve at which the protrusion height (x) is larger than its peak value of the distribution curve crosses the line of log 10  y=-18x+3.7,   (b) when the number of protrusions (y) on the film surface is at least 20/mm 2 , no protrusion exists which has a height (x) of at least 0.13 μm,   (c) said film has a surface roughness (Ra) of not more than 0.012 μm,   (d) said film has a Young's modulus in the longitudinal direction of at least 650 kg/mm 2 , and   (e) when heat-treated at 70° C. for 1 hour under no load, said film has a heat shrinkage in the longitudinal direction of not more than 0.1%.   
     
     
       2. The film of claim 1 wherein when the number of protrusions, y, is at least 20/mm 2 , the film does not contain protrusions having a height (x) of at least 0.10 micrometer. 
     
     
       3. The film of claim 1 which has a surface roughenss (Ra) of not more than 0.010 micrometer. 
     
     
       4. The film of claim 3 which has a surface roughness (Ra) of 0.004 to 0.008 micrometer. 
     
     
       5. The film of claim 3 which has a Young's modulus in the longitudinal direction of at least 700 kg/mm 2 . 
     
     
       6. The film of claim 5 which has a Young's modulus in the longitudinal direction of at least 750 kg/mm 2 . 
     
     
       7. The film of claim 1 which has a heat shrinkage in the longitudinal direction of not more than 0.06%. 
     
     
       8. The film of claim 7 which has a heat shrinkage in the longitudinal direction of not more than 0.03%. 
     
     
       9. The film of claim 7 which has been stretched to 4.1 to 5.5 times in the longitudinal direction and to 3.0 to 3.8 times in the transverse direction. 
     
     
       10. The film of claim 1 wherein the polyester is polyethylene terephthalate. 
     
     
       11. The film of claim 1 wherein the inert solid fine particles are fine particles of silicon dioxide, titanium dioxide or calcium carbonate. 
     
     
       12. The film of claim 1 which has a magnetic recording layer on its surface.